1. Field of the Invention
This invention relates generally to laser heads that are fiber coupled to laser diode pump sources, and more particularly to a laser or laser amplifier apparatus with a laser head that is fiber coupled to a diode source which produces a polarized pump beam, and includes a depolarization device or method which changes the polarized pump beam to a depolarized pump beam.
2. Description of Related Art
Laser systems with fiber coupling imaging are known. One such system includes a laser head coupled by an optical fiber or fiber bundle to a power supply which contains a laser diode pumping source suitable for pumping a solid state laser medium in the laser head. A pump beam is transmitted from the power supply to the laser head by the optical fiber. In many instances the image size from the fiber is matched to the mode size in the laser medium. The image size from the fiber is determined by the fiber diameter and divergence of the light from the fiber. The use of the fiber optic coupling allows the laser head to be compact and contain only the optical elements while all of the electronic and other elements, including the pumping source, can be placed in a separate, stationary power supply. Another advantage is that the cooling requirements of the head are minimized, allowing for high power, small air-cooled heads. Since the optical fiber can be long, this system configuration provides flexibility in the use of the laser, making the laser head portable. Further, different laser heads can be readily interchanged. Thus a variety of different laser heads which have different output characteristics can be used, essentially giving the user the benefit of several different systems but without the expense and redundance of entire separate systems because only a new laser head is required with the same power supply to have an entire new system.
Because the laser head contains only the optical components, the availability of different outputs becomes relatively economic. The down time in the case of a laser head failure is minimized since a replacement head can easily be substituted. Another advantage to the use of fiber optic coupling imagery for pumping the laser medium is that in the event the pump source must be replaced, the diode source can be replaced and matched into the fibers without the need for realignment of the laser head.
Diode pump sources typically produce a linearly polarized light output. Linear polarization is a condition in which the electric field vector associated with the light varies in amplitude at the light frequency, but is always oriented along one axis in space, in a plane perpendicular to the direction of light propagation. Passage of the light through subsequent optical elements can cause modification of this polarization. It may remain linear, but have a different direction within the perpendicular plane. The polarization may become circular, in which the vector amplitude remains constant, but its direction rotates at the light frequency, within this plane. More generally, the polarization can become elliptical, in which both the amplitude and direction of the electric vector vary within the perpendicular plane, at the light frequency, but with an arbitrary phase relationship.
Polarization is typically measured with a linear polarization analyzer, which measures the dominant direction of the polarization and the degree to which it approaches the ideal linear case. Results are frequently quoted as the angle of the dominant direction and the "extinction ratio", the ratio of power transmission through the analyzer with it oriented in the dominant direction and orthogonal to that direction. Perfect linear polarization would have an infinite extinction ratio, while perfect circular or perfectly random polarization would have a ratio of 1:1 and no dominant orientation. The general elliptical case would have a dominant direction and an intermediate extinction value.
Multimode optical fibers or fiber bundles, such as are commonly used in diode-pumped, fiber-coupled laser systems, are often viewed as "light pipes", which maintain the optical frequency and intensity of the light they transmit, but degrade its spacial and phase coherence. In particular, it was previously assumed that polarization of the input light would not be preserved at the output. However, it has been observed that in a short optical fiber or bundle, as is typically used in a fiber-coupled laser system, a significant amount of the input polarization character is preserved. For instance, linearly-polarized diode laser pump light, with an extinction ratio of perhaps 500:1 can, when passed through a short length of optical fiber, still exhibit extinction ratios of 3:1. However, the exact direction of polarization and extinction ratio vary among fibers and depend on the physical conditions surrounding the fiber. In particular, the output polarization varies with fiber position.
Pump light absorption characteristics in a solid state laser gain medium are often anisotropic, in other words, depend on the nature of the pump light polarization. Therefore, changes in this polarization can affect performance of the solid state laser. Based on the previous discussion, it should be clear that detailed performance of the laser can depend on the relative position of the power supply, fiber and laser head, and the particular fiber being used. This is a disadvantage when constant laser performance is desired when changing system components or repositioning the system.
To overcome this problem, pump light must be incident on the laser gain medium with a polarization state that is insensitive to laser system reconfiguration. This can be achieved if the pump light is completely depolarized, e.g., has no preferred orientation. Depolarization of the pump light can be accomplished between the laser diode and the fiber input along the fiber, or between the fiber output and the laser gain medium.
It would be desirable to provide a diode pumped, fiber coupled system that is insensitive to the movement of components of the system. It would be further desirable to provide a diode pumped, fiber coupled system which delivers an unpolarized pump beam to the laser gain medium.